Say the makers of a contract all die so no one left has a stake in or knows or cares about that contract. Say the contract never calls self-destruct on itself.

What happens to this old, unused contract? Does it burden the network that it exists? It should never be run or included in future blocks, but doesn't someone somewhere have to store it? Is there a way to reclaim this storage, like requiring all contracts to self destruct after X years?